Centralize version management in version.py and wire it through common.py and treeinfo.py

[guillermodotn]

Stacked on: #202

Summary

Makes productmd/version.py the single source of truth for all version
constants and utilities, replacing scattered hardcoded version logic across
the codebase.

Changes

• Rename DEFAULT_VERSION → OUTPUT_FORMAT_VERSION to clarify its role
  as the format version stamped into output files
• Enhance VersionedMetadataMixin: add _default_output_version class
  attribute for subclass overrides and get_output_version(force_version)
  for 3-level priority resolution (force > instance > class default)
• Wire common.py: replace hardcoded VERSION = (1, 2) with
  VERSION = OUTPUT_FORMAT_VERSION; use version_to_string /
  string_to_version in Header methods
• Wire treeinfo.py: pin Header.serialize() to VERSION_1_2,
  ensuring .treeinfo files always write v1.2 format regardless of the
  library default
• Update __init__.py: export Location, FileEntry, and
  OUTPUT_FORMAT_VERSION

[guillermodotn]

README.md road-map:

• Update VERSION to (2, 0) in productmd/common.py